This collaborative effort is designed to: 1. estimate the highest dose of daily hydroxyurea (HU) administration which can increase fetal hemoglobin production (Hb F) without toxicity in selected patients with sickle cell anemia (SS patients). 2. define some of the factors which determine the Hb F response to drug treatment. 3. determine relatively short-term toxicity of HU at doses considerably less than those used to treat myeloproliferative disorders. 4. The RFA does not contemplate a controlled trial. Therefore no data on clinical efficacy of HU as a treatment for SS disease will emerge during the relatively short study outlined here. Information will be collected on clinical responses to treatment, including frequency and duration of in- and outpatient painful crises, alterations in narcotic consumption during inpatient crises, frequency of documentable bacterial and viral infections during treatment, and alterations in immune status during HU therapy. It may be possible to relate changes in Hb F production to changes in clinical manifestations during therapy.